An incineration residue, for example, incineration ash, discharged from an incinerator for municipal refuse is treated for reduction in volume by melting in a melting furnace.
Conventionally, as one of such melting furnaces, a plasma melting furnace has been used. There are two types of plasma melting furnaces according to the disposition or the electrodes; a transfer type and a non-transfer type. A twin torch type out of the transfer type has an anode or a cathode installed in a torch and the other electrode installed outside the torch, e.g., on the bottom of a melting chamber. The non-transfer type has an anode and a cathode which are installed in one torch. The twin torch type has an anode and a cathode which are installed in each of a plurality of torches. Of these types, the twin torch type is the most superior in the maintenance and control of the electrodes.
And this twin torch type plasma melting furnace has anode and cathode torches of graphite disposed in the upper region of the melting chamber in the furnace body, and a molten base metal, which is an electric conductor, disposed in the bottom of the melting chamber. And plasma arcs are generated between the two electrode torches and the base metal to heat and melt incineration ash charged onto the base metal, the plasma arcs generated by these anodes and cathodes being substantially equally utilized.
In this connection, the plasma generating phenomenon at the cathode and anode torches is characterized in that the plasma at the anode having an inflow of electrons is less stable than the plasma at the cathode having an outflow of electrons. Therefore, when there is a large variation in the conditions for the furnace, e.g., when the furnace is started and hence plasma is started, during temperature rise or in the initial periods of the charging of a material to be melted (incineration ash) into the furnace, it is difficult to maintain the generation of plasma arcs; therefore, there has been a problem that the operation is intermittent.
Further, it is at the anode torch having an inflow of electrons rather than at the cathode torch having an outflow of electrons that the electrode tip is more heated. Therefore, in the case of an electrode of graphite, the anode torch tip is heated to a higher temperature, presenting a problem of severe electrode consumption.